This invention pertains to radio communications systems. In particular, this invention pertains to simultaneous broadcast or simulcast systems wherein a plurality of remotely sited transmitters simultaneously broadcast identical radio signals at a particular carrier frequency. Maximum signal coverage for a geographic area is provided by having one transmitter for each zone in the area. A problem with simulcast systems occurs however when a portable, transportable, hand-held or other type of mobile radio happens to be positioned between two or more transmitting sites such that it receives equal or nearly equal signal strength carrier signals from two or more transmitters. If the signals modulating the transmitters are of unequal amplitude, unequal phase delay, or unequal modulation the intelligibility of the message may be lost.
Prior art simulcast system inventions have addressed many problems of simulcast systems by including time delays between the program source and the transmitters. Other prior art inventions have provided for adjusting or modifying the signal modulating remote transmitters. Still other prior art systems have disclosed ways of synchronizing remote transmitters so that the broadcast signals from the transmitters are received substantially contemporaneously by a receiver in the field.
Prior art simulcast inventions have generally implemented such solutions using analog signal processing techniques. However, analog time delays, amplitude modulation and transmitter synchronization remedies suffer from the same short comings that make simulcast transmission difficult in and of itself. Analog signal processing techniques suffer from aging and instability and may require realignment by a service technician.
Digital techniques used with simulcast systems have generally been used only to transmit the signal for broadcast to the remote transmitters. Digital communications between the source of the signal for broadcast and the remote transmitters usually produces unacceptable reception by a receiver receiving nearly equal level signals from more than one transmitter. Prior art methods of digitizing of an analog signal using either Mu- or A-law companding, such as those used in standard telephone channel units, produce signal to noise and distortion ratios, (SINAD ratios) too low to produce acceptable modulation of multiple remote transmitters. Broadcast channel units which support higher bandwidth communications produce somewhat better results in simulcast systems but require multiple time slots in a DS-1 frame increasing the cost of a simulcast system.
Accordingly, there exists a need for an improved simulcast system capable of digitally transferring a signal for broadcast to remote transmitters using only a single DS-1 time slot. Such a simulcast system would preferably have SINAD ratios in excess of 40 dB. There also exists a need for a simulcast system capable of adjusting for time delay between each transmitter site and the programming source and be capable of adjusting the amplitude of the signal for broadcast to compensate for differences in the remote transmitters modulation characteristics as well as other equipment in the path to the transmitters.